The present invention relates to a system for controlling speed of an engine for a motor vehicle having a continuously variable belt-drive automatic transmission, and more particularly to a system for controlling engine speed for a motor vehicle having an automatic clutch and an antilock brake system.
It has become popular to provide an antilock brake system (hereinafter called ABS) on a motor vehicle in order to prevent wheels from locking when the vehicle is braked on a road having a low coefficient of friction, thereby improving a directional stability and maneuvering the vehicle. The ABS electronically controls a pressure of brake fluid and wheel speed to prevent the wheel from locking. However, in a vehicle provided with a continuously variable belt-drive transmission where a transmission ratio and line pressure are precisely controlled in accordance with wheel speed and other factors, the operation of the ABS much influences the control of the transmission. Hence it is preferable to control both the ABS and the transmission when the ABS is operated.
On the other hand, when the wheels are locked by rapid braking or wheel speed is rapidly decreased as is occurred in the operation of the ABS, it is necessary to disengage the automatic clutch in order to prevent the engine from stalling. When the clutch is disengaged, the engine speed decreases to an idle speed. As a result an output of an oil pump driven by the engine rapidly reduces, which causes decrease of the line pressure.
At the same time, oil pressure applied to a drive pulley of the continuously variable transmission decreases as well as the line pressure so that the belt slips on the pulleys. Thus, it is necessary to more function the oil pump.
Japanese Patent Application Laid-Open 62-247928 discloses an engine speed control system. In the system, when a driver selects an optimum transmission ratio, a throttle valve of the engine is controlled to maintain the engine speed to a set speed. However, the system is for controlling a cruise control system while the clutch is engaged. Thus, as the system is not for controlling the ABS, the system cannot be applied during the release of the clutch.